Various arrangements have been proposed for varying the speed-power relationship of boom movement in material handling machines of the type referred to above. Examples of known prior art arrangements are disclosed in Przybylski U.S. Pat. Nos. 3,388,819; 3,441,154 and 3,392,855. In these prior art patents, either the boom pivot or the boom cylinder pivot is pivotally carried on a control link which is in turn pivotally mounted on a boom support. Pivotal adjustment of the control link, and hence the speed-power relationship of boom movement, is achieved through the use of an additional piston-cylinder unit operatively connected between the boom support and the control link.
This type of arrangement is characterized by a number of disadvantages. For example, by interposing a control link between the boom support and either the boom pivot or boom cylinder pivot, the number of pins, pin bores and associates components is increased substantially, with a concomitant increase in the overall cost of the machine. This problem is further aggravated by the need to employ an additional piston-cylinder unit (with accompanying valve, hoses, etc.) to adjust the position of the control link and to hold it in place during operation of the machine. Such arrangements also are more susceptible to wear because of the increased "play" resulting from the multiple pivot points associated with each control link. An undesirable decrease in strength and rigidity also results from supporting either the boom pivot or boom cylinder pivot on a pivotal control link, which as compared to the heavier and more rugged boom support, is much less able to withstand bending and torsional stresses. The prior art pivotal control link arrangements also suffer from a lack of compactness, particularly where the boom pivots are link-mounted for adjustable movement beyond the confines of the boom support, as shown for example in U.S. Pat. No. 3,388,819.